A need exists for the development of small-molecule drugs designed to antagonize protein-protein interactions of therapeutic importance, as well as for the characterization of novel biological targets against which such drugs can effectively be directed. Yet few general strategies exist for the identification of novel therapeutic molecules and targets. Proteins belonging to the leucine-rich repeat (LRR) superfamily participate in diverse cellular processes that include signal transduction, cellular adhesion, tissue organization, hormone binding, RNA processing, and many others via molecular recognition mediated by a structurally conserved LRR interaction surface. The LRR binding surface merits in-depth study as a candidate molecular target for novel therapeutic reagents based on several factors: the broad functional diversity of LRR-containing proteins, the unique structure of the LRR motif itself, which could potentially be targeted with a high degree of specificity, and the growing number of LRR proteins linked to human pathologies including breast and other cancers. The focus of this study is to elucidate the molecular mechanisms by which the LRR recognizes and binds to its cognate ligands. The principal aim is to utilize a combinatorial random peptide aptamer library as an analytical tool to characterize the molecular basis for LRR-mediated binding interactions. The principal investigator intends to identify the complementarity-determining LRR residues of a 'typical' LRR protein, yeast CCR4, to define the peptide epitopes to which these residues bind, and to use a panel of CCR4-LRR mutations to define the precise LRR residues to which the aptamers bind. The principal investigator will also will isolate peptides specific for LRR-binding and utilize them as reagents to disrupt known LRR-mediated protein-protein interactions in vivo. These studies will provide a baseline for the future study and manipulation of other LRR containing target proteins of potential therapeutic interest.